Carbon nanotubes are substances having such a structure that a graphite sheet in which carbon atoms are arranged in a hexagonal network is cylindrically rolled; those which consist of one rolled layer are called a single-walled carbon nanotube, and those which consist of multiple rolled layers are called a multi-walled carbon nanotube. In particular, among the multi-walled carbon nanotubes, those which consist of two rolled layers are called a double-walled carbon nanotube. Carbon nanotubes themselves have excellent conductivity and are expected to be used as a conductive material.
In general, as a carbon nanotube, a single-walled carbon nanotube and a double-walled carbon nanotube with a small number of layers are known to have improved properties such as conductivity and thermal conductivity because they have a highly graphitic structure. However, these carbon nanotubes are likely to cause aggregation due to van der Waals force between adjacent carbon nanotubes and form a firm and very strong bundle structure composed of a plurality of carbon nanotubes. Consequently, the excellent conductivity and thermal conductivity that each carbon nanotube intrinsically has cannot be fully exerted, which is the biggest obstacle to industrial use of carbon nanotubes. To overcome such a high aggregability carbon nanotubes have, various dispersion methods have been proposed.
In Patent Document 1, as a method of dispersing carbon nanotubes in a solution, the method of adding single-walled carbon nanotubes into an amine solution and performing ultrasonication has been proposed.
Further, in Non-Patent Documents 1 and 2, not only ultrasonication, but also a dispersion method in which a surfactant is added to a solvent, and carbon nanotubes are covered by these substances, thereby improving the solvent-philicity of the carbon nanotubes has also been proposed. Although the surfactants used in these Non-Patent Documents 1 and 2 are various, it is reported that single-walled carbon nanotubes are dispersed by adding sodium dodecyl sulfate or sodium cholate as a surfactant to the single-walled carbon nanotubes in a water solvent and performing ultrasonication.